Seasonal variation in reproductive measures of tropical Roseate Terns Sterna dougallih previously undescribed breeding patterns in a seabird

Abstract

Seasonal variation in egg-laying, egg size, hatching success, hatchling mass, fledging success and chick growth of Roseate Terns Sterna dougallii breeding on Aride Island (Seychelles), Indian Ocean, were studied in 1997 and 1998. I investigated to what extent two patterns, common in a range of species, were followed by tropical Roseate Terns: (a) seasonal decrease in clutch size, egg size and breeding success and (b) an increase in breeding success with increasing egg weight. In 1997 (a poor year), the earliest nesting birds laid significantly smaller eggs, and chicks were lighter at hatching than those of peak nesting birds. The mean clutch size, of 1.04 eggs, showed no seasonal variation and no 'b'-eggs hatched. In 1998 (a good year) the earliest nesting birds laid eggs of similar size and their chicks were of similar weight to those of peak nesting birds. Mean clutch size, of 1.25 eggs, increased significantly through the season and about 60% of the 'b'-eggs hatched. In 1997, hatching success was 57% whereas in 1998 it was 80%. In both years, breeding success declined significantly through the season. The fact that the earliest breeding birds laid smaller eggs in a poor year and smaller clutches in a good year is in marked contrast t o a range of other species, and to temperate-nesting Roseate Terns. Egg volume explained about half of the variance in hatchling mass in both years, but only 15% of the variation

Influence of local and large-scale weather events and timing of breeding on tropical roseate tern reproductive parameters

“…Hatching success (percentage of eggs that hatched) and fledging success (percentage of hatched chicks that fledged) were obtained from nests mapped at the time of egg laying and monitored every 1 to 3 d from a permanent hide and/or vantage point (1993 to 2001), and from daily visits to study quadrats (1997 to 2001). Very similar values of productivity were obtained from both mapped and marked nests (Ramos 2001(Ramos , 2002b, and from marked/mapped nests versus estimated number of chicks fledged within the whole colonies (see 'Results', Table 3). Without disturbance, most chicks remained at their nest sites until they fledged or died (Ramos 2001).…”

“…Very similar values of productivity were obtained from both mapped and marked nests (Ramos 2001(Ramos , 2002b, and from marked/mapped nests versus estimated number of chicks fledged within the whole colonies (see 'Results', Table 3). Without disturbance, most chicks remained at their nest sites until they fledged or died (Ramos 2001). During daily visits of up to 30 min to the study quadrats (1997 to 2001), eggs were measured (length and breadth accuracy to 0.1 mm), chicks were ringed and weighed both at hatching and again on each encounter (Ramos 2001).…”

“…Without disturbance, most chicks remained at their nest sites until they fledged or died (Ramos 2001). During daily visits of up to 30 min to the study quadrats (1997 to 2001), eggs were measured (length and breadth accuracy to 0.1 mm), chicks were ringed and weighed both at hatching and again on each encounter (Ramos 2001). In 2-egg clutches, eggs were classified as 'a' (the first egg laid or the first egg to hatch) and 'b'.…”

“…When foraging costs are increased, the priority of adult breeding seabirds is to keep their body mass at a high level, at the expense of provisioning for their offspring (Weimerskirch et al 2000). Therefore, increasing levels of food stress are expected to reduce provisioning, lead to desertion of eggs and chicks, or for the birds to refrain from breeding (Chastrel et al 1995, Ramos 2001.…”

“…However, effects of food stress on breeding parameters is more marked in tropical seas due to strong food shortages during the breeding season (Ramos 2000(Ramos , 2002a. For example, mass mortality of chicks at an advanced stage of development, negative growth rates and low productivity are common for tropical roseate terns Sterna dougallii in the Great Barrier Reef, Pacific Ocean, and Aride Island of the Seychelles, Indian Ocean (Milton et al 1996, Ramos 2001.…”